1. Field of the Invention
The present invention relates to an imaging system and in particular to a charged particle multi-beamlet lithography system or inspection system.
2. Description of the Related Art
Currently, most commercial lithography systems use a mask as a means to store and reproduce the pattern data for exposing a target, such as a wafer with a coating of resist. In a maskless lithography system, beamlets of charged particles are used to write the pattern data onto the target. The beamlets are individually controlled, for example by individually switching them on and off, to generate the required pattern. For high resolution lithography systems designed to operate at a commercially acceptable throughput, the size, complexity, and cost of such systems becomes an obstacle.
Furthermore, existing charged particle beam technology is suitable for lithography systems for relatively course patterning of images, for example to achieve critical dimensions of 90 nm and higher. However, a growing need exists for improved performance. It is desired to achieve considerably smaller critical dimensions, for example 22 nm, while maintaining sufficient wafer throughput, e.g. between 10 and 100 wafers per hour. In order to achieve such a large throughput at ever decreasing feature sizes it is necessary to increase the current in the charged particle beam. An unwanted effect of this increase is that due to the higher current the interactions between the charged particles will increase leading to loss of resolution.
To counteract said interactions between the charged particles it is necessary to increase the field size of the system, that is the size of the area that is exposed in one cycle. In this manner, the interactions are reduced.
For such a system to provide accurate writing of a pattern on a target such as a wafer, it is essential that the charged particle beam is projected onto the target with highly controlled characteristics. Errors in the projection system will deteriorate the projected image. Throughout the charged particle optical system many factors contribute to errors in the pattern.
One such main contributor is the so-called collimator, which functions to create a substantially parallel charged particle beam from the diverging beam emanating from the charged particle source.
Known systems often apply electrostatic lenses comprising multiple electrodes for the focusing or collimating of a charged particle beam. It is a known problem with this type of lens that it suffers from spherical and chromatic aberrations, introduced by the electrodes of the lens, reducing the quality of the projected images. When increasing the field size of the exposure, these aberrations only increase.
As such the known lenses typically allow for some control of aberrations or beam uniformity, however these cannot provide both requirements to the level required by high-end charged particle lithography. The known designs cannot reliably and cost effectively provide the reduction in critical dimensions, as required by the roadmap for future development of the lithography industry. To maintain a commercially feasible throughput, the reduction in critical dimensions typically necessitates an increase in charged particles that can be deposited on the target. One way of achieving this increase is massively increasing the number of beamlets used. This is not possible with the known designs.
One such system disclosed in WO2007/013802 comprises a charged particle column operating in vacuum with a charged particle source including a charged particle extraction means, a means for creating a plurality of parallel beamlets from the extracted charged particles and a plurality of electrostatic lens structures comprising electrodes. The electrostatic lens structures serve amongst other the purpose of focussing and blanking the beamlets. Blanking in this system is realised by deflecting one or a multiplicity of such usually focused charged particle beams to prevent the particle beam or multiplicity of beamlets from reaching the target such as a wafer. For realizing the final part of the projection on the target of a computer based image pattern, non-blanked beamlets are, at a final set of such electrostatic lenses deflected in a so-called writing direction, thereby scanned over the target surface, as part of the imaging process of the target.
Another charged particle system is known from WO2010/037832, relating to a charged particle lithography system comprising an electrostatic lens or lens array, describing a subdivision of the projection field into areas comprising apertures which are used for letting charged particle beams pass, apertures not used for letting beams pass, and bars which do not comprise apertures, but are included for mechanical strength.
A solution is therefore needed which can be scaled up to the required massive amounts of parallel charged particle beams whilst reducing aberrations to levels suited for high-end charged particle lithography.